AFFINE CIPHER

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The affine cipher is the multiplicative cipher, which is then encrypted by the Caesar cipher on top of that. The affine cipher needs two keys: one for the multiplicative cipher multiplication and the other for the Caesar cipher addition. For the affine cipher program, we will use a single integer for the key. We will use some simple math to split this key into the two keys, which we will call Key A and Key B.Its working needs to explained . If you want me to explain how it works ,let me know in the comments and I'll explain it .

First of all we need to make a python file and save it as cryptomath.We will need to call this later in our program.

now create a new folder and save the cryptomath and affine cipher there.

NOTE :-  both the python file should be in the same folder or it would be diificult to call the crpytomath module later in our cipher.

SOURCE CODE (Cryptomath) :-

                                                                                                                   source- www.inventwithpython.com

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# Cryptomath Module

def gcd(a, b):
    # Return the GCD of a and b using Euclid's Algorithm
    while a != 0:
        a, b = b % a, a
    return b


def findModInverse(a, m):
    # Returns the modular inverse of a % m, which is
    # the number x such that a*x % m = 1

    if gcd(a, m) != 1:
        return None # no mod inverse if a & m aren't relatively prime

    # Calculate using the Extended Euclidean Algorithm:
    u1, u2, u3 = 1, 0, a
    v1, v2, v3 = 0, 1, m
    while v3 != 0:
        q = u3 // v3 # // is the integer division operator
        v1, v2, v3, u1, u2, u3 = (u1 - q * v1), (u2 - q * v2), (u3 - q * v3), v1, v2, v3
    return u1 % m

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After you've created the cryptomath, proceed to the cipher.

The source code for the same is below.

SOURCE CODE :-

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import sys, pyperclip, cryptomath, random
SYMBOLS = """ !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~""" # note the space at the front


def main():
    myMessage = str(input("ENTER YOUR MESSAGE NOW"))
    myKey = int(input("ENTER YOUR KEY"))
    myMode = 'encrypt' # set to 'encrypt' or 'decrypt'

    if myMode == 'encrypt':
        translated = encryptMessage(myKey, myMessage)
    elif myMode == 'decrypt':
        translated = decryptMessage(myKey, myMessage)
    print('Key: %s' % (myKey))
    print('%sed text:' % (myMode.title()))
    print(translated)
    pyperclip.copy(translated)
    print('Full %sed text copied to clipboard.' % (myMode))


def getKeyParts(key):
    keyA = key // len(SYMBOLS)
    keyB = key % len(SYMBOLS)
    return (keyA, keyB)


def checkKeys(keyA, keyB, mode):
    if keyA == 1 and mode == 'encrypt':
        sys.exit('The affine cipher becomes incredibly weak when key A is set to 1. Choose a different key.')
    if keyB == 0 and mode == 'encrypt':
        sys.exit('The affine cipher becomes incredibly weak when key B is set to 0. Choose a different key.')
    if keyA < 0 or keyB < 0 or keyB > len(SYMBOLS) - 1:
        sys.exit('Key A must be greater than 0 and Key B must be between 0 and %s.' % (len(SYMBOLS) - 1))
    if cryptomath.gcd(keyA, len(SYMBOLS)) != 1:
        sys.exit('Key A (%s) and the symbol set size (%s) are not relatively prime. Choose a different key.' % (keyA, len(SYMBOLS)))


def encryptMessage(key, message):
    keyA, keyB = getKeyParts(key)
    checkKeys(keyA, keyB, 'encrypt')
    ciphertext = ''
    for symbol in message:
        if symbol in SYMBOLS:
            # encrypt this symbol
            symIndex = SYMBOLS.find(symbol)
            ciphertext += SYMBOLS[(symIndex * keyA + keyB) % len(SYMBOLS)]
        else:
            ciphertext += symbol # just append this symbol unencrypted
    return ciphertext


def decryptMessage(key, message):
    keyA, keyB = getKeyParts(key)
    checkKeys(keyA, keyB, 'decrypt')
    plaintext = ''
    modInverseOfKeyA = cryptomath.findModInverse(keyA, len(SYMBOLS))

    for symbol in message:
        if symbol in SYMBOLS:
            # decrypt this symbol
            symIndex = SYMBOLS.find(symbol)
            plaintext += SYMBOLS[(symIndex - keyB) * modInverseOfKeyA % len(SYMBOLS)]
        else:
            plaintext += symbol # just append this symbol undecrypted
    return plaintext


def getRandomKey():
    while True:
        keyA = random.randint(2, len(SYMBOLS))
        keyB = random.randint(2, len(SYMBOLS))
        if cryptomath.gcd(keyA, len(SYMBOLS)) == 1:
            return keyA * len(SYMBOLS) + keyB


# If affineCipher.py is run (instead of imported as a module) call
# the main() function.
if __name__ == '__main__':
    main()

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                                                                                                                            source- www.inventwithpython.com